Method of fabricating semiconductor memory device having enlarged cell contact area

ABSTRACT

A semiconductor substrate is provided. Active areas and trench isolation regions are formed. The active areas extend along a first direction. Buried word lines extending along a second direction are formed in the semiconductor substrate. Two of the buried word lines intersect with each of the active areas, separating each of the active areas into a digit line contact area and two cell contact areas. Buried digit lines extending along a third direction are formed above the buried word lines. An upper portion of the trench isolation region is removed to form an L-shaped recessed area around each of the cell contact areas. The L-shaped recessed area exposes sidewalls of the cell contact areas. An epitaxial silicon growth process is then performed to grow an epitaxial silicon layer from the exposed sidewalls and a top surface of each of the cell contact areas, thereby forming enlarged cell contact areas.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional of U.S. patent application Ser. No. 15/002,401,filed Jan. 21, 2016, pending, the disclosure of which is herebyincorporated herein in its entirety by this reference.

TECHNICAL FIELD

This invention relates generally to semiconductor memory devices and amethod of fabricating the same. More particularly, the present inventionrelates to a memory device including buried (or damascened) digit lines(BDLs)/buried word lines (BWLs), as well as enlarged cell contact areas,in the cell array, and a method of fabricating the same.

BACKGROUND

As known in the art, a dynamic random access memory (DRAM) device ismade up of memory cells. Each cell of a DRAM device comprises atransistor and a capacitor electrically coupled to a terminal such asthe drain (or source) of the transistor. A digit line is electricallycoupled to another terminal such as the source (or drain) of thetransistor. The memory cells are addressed via a word line and the digitline, one of which addresses a “column” of memory cells while the otheraddresses a “row” of the memory cells.

One type of the typical DRAM device utilizes buried word line (BWL)architecture comprising parallel word lines embedded in a cell array.The buried word lines are fabricated in word line trenches thatintersect with the active areas (AAs). The capacitor is stacked on amajor surface of the silicon substrate and the digit line is constructedover the major surface of the silicon substrate and over the capacitor.

As the size of DRAM cell shrinks, the surface area of the AA becomessmaller and smaller. The decreasing surface area of the AAs results ininsufficient cell contact area (or landing area) for the capacitors anddecreased process window when forming a cell contact layer (or landingpad). Additionally, there is a continuing goal to further decrease thecell area. Therefore, it has become a major issue in this technicalfield to cope with the insufficient cell contact area and narrow processwindow.

BRIEF SUMMARY

It is one object of the invention to provide an improved DRAM devicecomprised of a plurality of memory cells having an effective cell sizeof 6F² and an enlarged cell contact area.

It is another object of the invention to provide an improved DRAM devicehaving buried digit lines/word lines and a capacitor-over-digit linestructure.

It is another object of the invention to provide a method forfabricating a DRAM device without the need of forming a cell contactlayer or a landing pad.

According to one embodiment of the invention, a method for fabricating amemory array is disclosed. A semiconductor substrate is provided. Aplurality of active areas and a trench isolation region isolating theplurality of active areas from one another are formed. The active areasextend along a first direction. Buried word lines extending along asecond direction are formed in the semiconductor substrate. Two of theburied word lines intersect with each of the active areas, separatingeach of the active areas into three portions including a digit linecontact area and two cell contact areas. The second direction is notperpendicular to the first direction. Buried digit lines extending alonga third direction in the semiconductor substrate are formed above theburied word lines. The third direction is substantially perpendicular tothe second direction. An upper portion of the trench isolation region isselectively removed to form an L-shaped recessed area around each of thetwo cell contact areas. The L-shaped recessed area exposes sidewalls ofeach of the two cell contact areas. An epitaxial silicon growth processis then performed to grow an epitaxial silicon layer from the exposedsidewalls and a top surface of each of the cell contact areas, therebyforming enlarged cell contact areas.

According to one aspect of the invention, a memory array is disclosed.The memory array includes a semiconductor substrate having thereon aplurality of active areas and a trench isolation region between theactive areas. The active areas extend along a first direction. Buriedword lines extend along a second direction in the semiconductorsubstrate. Two of the buried word lines intersect with each of theactive areas, separating each of the active area into three portionsincluding a digit line contact area and two cell contact areas. Thesecond direction is not perpendicular to the first direction. Burieddigit lines extend along a third direction in the semiconductorsubstrate above the buried word lines. The third direction issubstantially perpendicular to the second direction. An epitaxialsilicon layer extends from exposed sidewalls and a top surface of eachof the cell contact areas.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the embodiments, and are incorporated in and constitutea part of this specification. The drawings illustrate some of theembodiments and, together with the description, serve to explain theirprinciples. In the drawings:

FIGS. 1A-7C are schematic diagrams illustrating a method for fabricatinga memory device with buried digit lines and buried word lines integratedin a memory array of the memory device in accordance with one embodimentof the present invention, wherein:

FIGS. 1A-7A are top views of schematic layout diagrams of memory arrayof the memory device in different manufacturing stages according to anexemplary embodiment of the invention; and

FIGS. 1B-7B and 1C-7C are schematic, cross-sectional views taken alonglines I-I′ and II-II′, respectively, in the layout diagrams depicted inFIGS. 1A-7A.

It should be noted that all the figures are diagrammatic. Relativedimensions and proportions of parts of the drawings have been shownexaggerated or reduced in size, for the sake of clarity and conveniencein the drawings. The same reference numerals are generally used to referto corresponding or similar features in modified and differentembodiments.

DETAILED DESCRIPTION

In the following detailed description of the disclosure, reference ismade to the accompanying drawings, which form a part hereof, and inwhich is shown, by way of illustration, specific embodiments in whichthe invention may be practiced. These embodiments are described insufficient detail to enable those skilled in the art to practice theinvention. Other embodiments may be utilized and structural changes maybe made without departing from the scope of the present disclosure.

The following detailed description is, therefore, not to be taken in alimiting sense, and the scope of the present invention is defined onlyby the appended claims, along with the full scope of equivalents towhich such claims are entitled. One or more implementations of thepresent invention will now be described with reference to the attacheddrawings, wherein like reference numerals are used to refer to likeelements throughout, and wherein the illustrated structures are notnecessarily drawn to scale.

The terms “wafer” and “substrate” used herein include any structurehaving an exposed surface onto which a layer is deposited according tothe present invention, for example, to form the integrated circuit (IC)structure. The term “substrate” is understood to include semiconductorwafers. The term “substrate” is also used to refer to semiconductorstructures during processing, and may include other layers that havebeen fabricated thereupon. Both wafer and substrate include doped andundoped semiconductors, epitaxial semiconductor layers supported by abase semiconductor or insulator, as well as other semiconductorstructures well known to one skilled in the art.

The term “horizontal” as used herein is defined as a plane parallel tothe conventional major plane or surface of the semiconductor substrate,regardless of its orientation. The term “vertical” refers to a directionperpendicular to the term horizontal as just defined. Terms, such as“on,” “above,” “below,” “bottom,” “top,” “side” (as in “sidewall”),“higher,” “lower,” “over,” and “under,” are defined with respect to thehorizontal plane.

The present invention pertains to an improved DRAM device that iscomprised of a plurality of memory cells having an effective cell sizeof 6F² (e.g., 3F×2F) and an enlarged cell contact area. The enlargedcell contact area involves the use of inventive self-constrainedepitaxial growth technology, which effectively avoids shorting betweenneighboring cells.

The width of the feature is also referred to as the CD or minimumfeature size (“F”) of the line. The CD is typically the smallestgeometrical feature, such as the width of an interconnect line, contact,or trench, that is formed during IC manufacturing using a giventechnology, such as photolithography.

Please refer to FIGS. 1A, 1B and 1C. FIG. 1A is a top view of theschematic layout of the memory array of the memory device after theformation of columns of buried word lines (BWLs) according to oneembodiment of the invention. FIGS. 1B and 1C are schematic,cross-sectional views taken along lines I-I′ and II-II′, respectively,in FIG. 1A. First, a semiconductor substrate 10 such as a silicon waferis provided. A plurality of active areas 12 is formed in thesemiconductor substrate 10. Shallow trench isolation (STI) structures 14are provided between the active areas 12 to isolate active areas 12 fromone another. The formation of the STI structures 14 is known in the art.However, it is noted that the STI structure described herein may includeadditional features than those known in the art. For example, usingconventional lithographic processes, a photoresist pattern (not shown)may be formed on the semiconductor substrate 10, which defines trenchpatterns to be etched into the semiconductor substrate 10. Using thephotoresist pattern as a hard mask, a dry etching process is performedto etch the semiconductor substrate 10 to thereby form trenches. Thetrenches are then filled with insulating materials such as siliconoxide. The longitudinal direction of each active area 12 extends along areference AA direction. Each active area 12 has a longer side that is inparallel with the longitudinal direction of each active area 12. Anincluded angle (acute angle) 0 between the reference AA direction and areference x-axis direction may range between 15° and 60°, but should notbe limited thereto.

After the formation of the STI structures 14 and the active areas 12,columns of line-shaped buried word lines 16 are fabricated in thesemiconductor substrate 10. As can be seen in FIG. 1A, the columns ofline-shaped buried word lines 16 extend along a reference y-axis, andtwo buried word lines 16 intersect with each active area 12, therebyseparating each active area into three portions including a digit linecontact area 12 a and two cell contact areas (or capacitor landingareas) 12 b. As can be best seen in FIG. 1A, the two cell contact areas12 b are located at two distal ends of each active area 12, and thedigit line contact area 12 a is between two line-shaped buried wordlines 16.

As best seen in FIG. 1B, each of the buried word lines 16 includes aconductive portion 162 embedded at a lower portion of a word line trench160. The conductive portion 162 may comprise a layer of metal, metalcomposite or layers of conductive materials. For example, the conductiveportion 162 may comprise titanium nitride (TiN), titanium/titaniumnitride (Ti/TiN), tungsten nitride (WN), tungsten/tungsten nitride(W/WN), tantalum nitride (TaN), tantalum/tantalum nitride (Ta/TaN),titanium silicon nitride (TiSiN), tantalum silicon nitride (TaSiN), andtungsten silicon nitride (WSiN), or a combination thereof. Theconductive portion 162 is encapsulated by an insulating layer 164 suchas silicon oxide lining an interior surface of the word line trench 160and a cap layer 166 situated atop the conductive portion 162. The caplayer 166 has a top surface that is flush with a top surface 10 a of thesemiconductor substrate 10. For example, the cap layer 166 may comprisesilicon nitride, but is not limited thereto.

Please refer to FIGS. 2A, 2B and 2C. FIG. 2A is a top view of theschematic layout of the memory array of the memory device after theformation of buried digit line (BDL) trenches according to oneembodiment of the invention. FIGS. 2B and 2C are schematic,cross-sectional views taken along lines I-I′ and II-II′, respectively,in FIG. 2A. As shown in FIG. 2A, rows of BDL trenches 22 are formed andare recessed into the top surface 10 a of the semiconductor substrate10. The rows of BDL trenches 22 extend along the reference x-axis andintersect with the active areas 12 at the included angle θ, therebyexposing the digit line contact area 12 a of each active area 12. Asshown in FIG. 2B, the depth of each of the BDL trenches 22 is wellcontrolled such that the conductive portion 162 of each buried word line16 is not exposed. Subsequently, a conformal liner layer 210 such as asilicon nitride liner is blanket-deposited into each BDL trench 22, butdoes not completely fill up the BDL trench 22. The liner layer 210 maybe deposited using chemical vapor deposition (CVD) or atomic layerdeposition (ALD) methods, but is not limited thereto. In someembodiments, the liner layer 210 may cover the area outside the BDLtrenches 22.

Please refer to FIGS. 3A, 3B and 3C. FIG. 3A is a top view of theschematic layout of the memory array of the memory device after theformation of digit line contact openings in the photoresist layeraccording to the embodiment of the invention. FIGS. 3B and 3C areschematic, cross-sectional views taken along lines I-I′ and II-II′,respectively, in FIG. 3A. As shown in FIGS. 3A, 3B and 3C, a photoresistlayer 30 is formed over the semiconductor substrate 10. A plurality ofopenings 302 is formed in the photoresist layer 30 to expose portions ofthe liner layer 210 in respective digit line contact areas 12 a wherethe line-shaped BDL trenches 22 intersect with each of the active areas12. The openings 302 are aligned with the digit line contact areas 12 ato thereby expose only the portions of the liner layer 210, which aresituated directly above the digit line contact areas 12 a. Subsequently,an etching process is performed to etch away the exposed portions of theliner layer 210 through the openings 302, thereby exposing surfaces ofthe semiconductor substrate 10 within the digit line contact areas 12 a.The remaining photoresist layer 30 is removed.

Please refer to FIGS. 4A, 4B and 4C. FIG. 4A is a top view of theschematic layout of the memory array of the memory device after fillingthe BDL trenches 22 with metal according to the embodiment of theinvention. FIGS. 4B and 4C are schematic, cross-sectional views takenalong lines I-I′ and respectively, in FIG. 4A. As shown in FIGS. 4A, 4Band 4C, after etching the liner layer 210 through the openings 302, ametal layer 220 comprising, for example, Ti, TiN or W, is deposited intothe BDL trenches 22. The metal layer 220 is insulated from the activeareas 12 by the liner layer 210 except for the surface of thesemiconductor substrate 10 within the digit line contact areas 12 a. Ascan be seen in FIGS. 4B and 4C, the metal layer 220 is electricallyconnected to the semiconductor substrate 10 within the digit linecontact areas 12 a. According to the embodiment, the BDL trenches 22 arecompletely filled up with the metal layer 220. By performing suitableprocesses such as etching or polishing, a top surface of the metal layer220 is flush with the top surface 10 a of the substrate 10, the topsurface of the cap layer 166 and the top surface of the STI structures14.

Please refer to FIGS. 5A, 5B and 5C. FIG. 5A is a top view of theschematic layout of the memory array of the memory device after forminga cap layer 230 atop the metal layer 220 according to the embodiment ofthe invention. FIGS. 5B and 5C are schematic, cross-sectional viewstaken along lines I-I′ and respectively, in FIG. 5A. As shown in FIGS.5A, 5B and 5C, the top surface of the metal layer 220 is recessed to alower level that is lower than the top surface 10 a of the semiconductorsubstrate 10. The metal layer 220 is capped with a cap layer 230. Forexample, the cap layer 230 may be a silicon nitride cap layer, but isnot limited thereto. To form the cap layer 230, a silicon nitride layer(not shown) may be deposited over the semiconductor substrate 10 in ablanket manner. The silicon nitride layer completely fills up the recessabove the metal layer 220. A chemical-mechanical polishing (CMP) processmay be performed to remove excess silicon nitride layer outside the BDLtrenches 22.

Further, in FIG. 5A, a plurality of square areas 40 are indicated. Eachof the square areas 40 is surrounded by the SiN liner layer 210 and SiNcap layer 230 in the BDL trenches 22 in the reference x-axis direction,and the SiN cap layer 166 in the reference y-axis direction. In eachsquare area 40, the cell contact area 12 b of the active area 12 isexposed. It is desirable to provide a larger cell contact area in orderto improve the cell contact Rc. According to the embodiment, the SiNsurrounded square areas 40 constitute self-constrained epitaxial growthregions for following cell contact area enlargement.

Please refer to FIGS. 6A, 6B and 6C. FIG. 6A is a top view of theschematic layout of the memory array of the memory device after oxiderecess and epitaxial growth in the square areas 40, according to theembodiment of the invention. FIGS. 6B and 6C are schematic,cross-sectional views taken along lines I-I′ and II-II′, respectively,in FIG. 6A. As shown in FIGS. 6A, 6B and 6C, after forming the cap layer230, an etching (oxide recess) process is performed to selectively etchaway an upper portion of the STI structures 14 from each of the squareareas 40. During the oxide recess process, the silicon oxide layer ofthe STI structures 14 may be etched by using, for example, diluted HF(DHF) or the like, which is selective to the surrounding silicon nitridecap layers and silicon. However, it is understood that the selectiveetching of the silicon oxide layer of the STI structures 14 may becarried out using other suitable methods, for example, a dry etchingprocess.

As can be seen in FIGS. 6A and 6C, after the oxide recess process, anL-shaped recessed area 420 having a step height h is formed within eachof the square areas 40. After removing the upper (oxide) portion of theSTI structures 14 from each of the square areas 40, two adjacentsidewalls 122 and 124 of the cell contact area 12 b (FIG. 5A) of theactive area 12 are exposed. Subsequently, an epitaxial silicon growthprocess is performed to grow an epitaxial silicon layer 52 from theexposed cell contact area 12 b (FIG. 5A) and the sidewalls 122 and 124,thereby forming an enlarged cell contact area 12 b′. The epitaxialsilicon layer 52 may function as a landing pad for a capacitor. Theepitaxial silicon growth is confined to each square area 40 that issurrounded by the SiN cap layers extending along the reference x-axisdirection and the reference y-axis direction. Therefore, shortingbetween adjacent cells can be avoided. It is understood that prior tothe epitaxial silicon growth process, the major surface 10 a of thesemiconductor substrate 10 may be subjected to a pre-clean process.

According to the embodiment, the L-shaped recessed area 420 is notfilled up with the epitaxial silicon layer 52, leaving a gap between theepitaxial silicon layer 52 and the adjacent BDL trenches 22 and the BWLtrenches 160 (FIG. 6B). However, it is understood that in otherembodiments, the L-shaped recessed area 420 may be completely filled upwith the epitaxial silicon layer.

Please refer to FIGS. 7A, 7B and 7C. FIG. 7A is a top view of theschematic layout of the memory array of the memory device after forminga dielectric stack and capacitors according to the embodiment of theinvention. FIGS. 7B and 7C are schematic, cross-sectional views takenalong lines I-I′ and II-II′, respectively, in FIG. 7A. As shown in FIGS.7A, 7B and 7C, after performing the epitaxial silicon growth process, adielectric stack 70 may be deposited over the major surface 10 a of thesemiconductor substrate 10. For example, the dielectric stack 70 mayinclude, but is not limited to, an etch stop layer 71, an inter-layerdielectric 72, an intermediate layer 73, an inter-layer dielectric 74,and a cap layer 75. For example, the etch stop layer 71 may comprisesilicon nitride, but is not limited thereto. The inter-layer dielectric72 and the inter-layer dielectric 74 may comprise phosphosilicate glass(PSG), borophosphosilicate glass (BPSG), silicon oxide, or low-kmaterials, but is not limited thereto. The cap layer 75 may comprisesilicon nitride or silicon oxy-nitride, but is not limited thereto.

As shown in FIG. 7C, the etch stop layer 71 conformally covers the caplayer 166, the exposed surface of the liner layer 210 within theL-shaped recessed area 420, the epitaxial silicon layer 52, and the STIstructure 14. After forming the dielectric stack 70, capacitor trenches810 are formed in the dielectric stack 70 by using, for example, a dryetching process. A bottom of each capacitor trench 810 exposes a portionof each epitaxial silicon layer 52. A capacitor 80 is then formed withineach capacitor trench 810. The capacitor 80 may comprise a bottomelectrode, a capacitor dielectric layer, and a top electrode. Thedetailed structure of the capacitor 80 is not explicitly shown.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

1. A method for fabricating a memory array, comprising: providing asemiconductor substrate having thereon a plurality of active areas and atrench isolation region isolating the plurality of active areas from oneanother, wherein the active areas extend along a first direction;forming buried word lines extending along a second direction in thesemiconductor substrate, wherein two of the buried word lines intersectwith each of the active areas, separating each of the active areas intothree portions including a digit line contact area and two cell contactareas, wherein the second direction is not perpendicular to the firstdirection; forming buried digit lines extending along a third directionin the semiconductor substrate above the buried word lines, wherein thethird direction is substantially perpendicular to the second direction;selectively removing an upper portion of the trench isolation region toform an L-shaped recessed area around each of the two cell contactareas, thereby exposing sidewalls of each of the two cell contact areas;and performing an epitaxial silicon growth process to grow an epitaxialsilicon layer from the exposed sidewalls and a top surface of each ofthe cell contact areas, thereby forming enlarged cell contact areas. 2.The method for fabricating a memory array according to claim 1, whereineach of the buried word lines comprises a conductive portion, a firstcap layer situated atop the conductive portion, and an insulating layerbetween the conductive portion and the semiconductor substrate.
 3. Themethod for fabricating a memory array according to claim 2, whereinforming the buried digit lines extending along the third direction inthe semiconductor substrate comprises: recessing line-shaped burieddigit line (BDL) trenches into the semiconductor substrate; blanketdepositing a liner layer over the semiconductor substrate and into theline-shaped BDL trenches; removing a portion of the liner layer from thedigit line contact area where the line-shaped BDL trenches intersectwith each of the active areas; depositing a metal layer into theline-shaped BDL trenches; and capping the metal layer by a second caplayer.
 4. The method for fabricating a memory array according to claim3, wherein the buried digit line trenches do not expose the conductiveportion of each of the buried word lines.
 5. The method for fabricatinga memory array according to claim 3, wherein the liner layer comprisessilicon nitride.
 6. The method for fabricating a memory array accordingto claim 3, wherein the first cap layer extends along the firstdirection and the second cap layer extends along the second direction,and the first and second cap layers confine the epitaxial silicon layer.7. The method for fabricating a memory array according to claim 3,wherein the first cap layer and the second cap layer are both comprisedof silicon nitride.
 8. The method for fabricating a memory arrayaccording to claim 1, wherein the epitaxial silicon layer does notcompletely fill up the L-shaped recessed area.
 9. The method forfabricating a memory array according to claim 1, wherein an includedangle between the first direction and the third direction ranges between15° and 60°.